1. Field of the Invention
This invention relates to improvements in a camshaft of a valve operating system for an internal combustion engine, and more particularly to such a camshaft which undergoes remelting treatment to form a hardened layer at the surface section thereof.
2. Description of the Prior Art
Camshafts of a valve operating system for an internal combustion engine are required to be high in wear resistance particularly at the cam surface which is slidably contactable with an opposite member or a rocker arm. In order to improve the wear resistance, it has been proposed and put into practical use to set a chilling block at a part of a die for casting the camshaft, so that super cooling is made to a part (in contact with the chilling block) of the die thereby forming a chilled hardened layer in the casting. However, according to this method, setting the chilling block is troublesome while increasing production of casting fin or burr thereby requiring removing steps therefor.
As other methods for forming the chilled hardened layer, there have been proposed induction hardening and remelting treatment which is carried out by remelting the surface of the camshaft by high density heat energy such as TIG (tungsten inert gas) arc and thereafter by allowing the camshaft to be self-cooled. A variety of such conventional remelting treatments have been proposed to form the chilled hardened layer in the camshaft as listed below: 1) Remelting treatment by plasma arc is made to the surface of the camshaft whose material is gray cast iron essentially consisting of carbon (C) in an amount ranging from 3.0 to 3.6% by weight, silicon (Si) in an amount ranging from 1.5 to 2.4% by weight, phosphorus (P) in an amount not more than 0.1% by weight, manganese (Mn) in an amount ranging from 0.08 to 0.2% by weight, and balance being iron (Fe) as disclosed in Japanese Patent Provisional Publication No. 60-184694. 2) A chilled hardened layer whose major phase is cementite is formed throughout the whole periphery of the cam surface under remelting treatment and self-cooling as disclosed in Japanese Patent Provisional Publication No. 60-234167). 3) A chilled hardened layer (having a major phase of cementite) and a quench hardened layer are formed throughout the whole periphery of the cam surface as disclosed in Japanese Patent Provisional Publication No. 60-234169. 4) A chilled hardened layer (having a major phase of cementite) occupies 10 to 75% of the whole cam surface, while quench hardening is made to the remaining cam surface, as disclosed in Japanese Patent Provisional Publication No. 60-258426. Other similar techniques have been developed as disclosed in Japanese Patent Provisional Publication Nos. 55-148772, 60-234168, and 61-522.
However, difficulties have been encountered in such conventional techniques to form a chilled hardened layer. In other words, in the conventional techniques, a mixed structure of bainite and troostite cannot be formed immediately below and in contact with the chilled layer for the reasons of no control of cooling rate made after the remelting treatment, no preheating of the camshaft material made prior to the remelting treatment, and the like. It is confirmed that the camshaft is low in pitting resistance during engine running if the layer in contact with the chilled layer is formed of only bainite transformation phase. No control of cooling rate after the remelting treatment also leads to a problem that defects are produced in the chilled structure. No control of preheating also leads to the problem of production of cracks after finish polishing and formation of pearlite structure which causes a low pitting resistance. Additionally, in order to control the cooling rate after the remelting treatment, a special facility therefor is required thereby increasing production cost.